Drilling with mixed tooth types

ABSTRACT

A rotary-cutter Earth-penetrating bit in which a given track of bottom hole is attacked by inserts of two different types, e.g. chisel-shaped and pointed, wider and narrower, higher and shorter, and/or of different materials. The different types of inserts can both be included within a single row of a cone.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to earth-penetrating drill bits, andparticularly to rotary-cone rotating bits such as are used for drillingoil and gas wells.

Background: Rotary Drilling

Oil wells and gas wells are drilled by a process of rotary drilling. Ina conventional drill rig, as seen in Figure ? a drill bit 50 is mountedon the end of a drill string 52, made of many sections of drill pipe,which may be several miles long. At the surface a rotary drive turns thestring, including the bit at the bottom of the hole, while drillingfluid (or “mud”) is pumped through the string by very powerful pumps 54.

The bit's teeth must crush or cut rock, with the necessary forcessupplied by the “weight on bit” (WOB) which presses the bit down intothe rock, and by the torque applied at the rotary drive. While the WOBmay in some cases be 100,000 pounds or more, the forces actually seen atthe drill bit are not constant: the rock being cut may have harder andsofter portions (and may break unevenly), and the drill string itselfcan oscillate in many different modes. Thus the drill bit must be ableto operate for long periods under high stresses in a remote environment.

When the bit wears out or breaks during drilling, it must be brought upout of the hole. This requires a process called “tripping”: a heavyhoist pulls the entire drill string out of the hole, in stages of (forexample) about ninety feet at a time. After each stage of lifting, one“stand” of pipe is unscrewed and laid aside for reassembly (while theweight of the drill string is temporarily supported by anothermechanism). Since the total weight of the drill string may be hundredsof tons, and the length of the drill string may be tens of thousands offeet, this is not a trivial job. One trip can require tens of hours andis a significant expense in the drilling budget. To resume drilling theentire process must be reversed. Thus the bit's durability is veryimportant, to minimize round trips for bit replacement during drilling.

Background: Drill Bits

One of the most important types of rotary drill bits commonly used indrilling for oil and gas is the roller cone bit, seen in FIG. 6. In suchbits, a rotating cone 82 with teeth 84 on its outer surface is mountedon an arm 46 of the drill bit body. The arms 46 (typically three) extenddownhole from the bit body, and each carries a spindle on which the coneis mounted with heavy-duty bearings. The support arms are roughlyparallel to the drill string, but the spindles are angled to pointradially inward and downhole.

As the drill bit rotates, the roller cones roll on the bottom of thehole. The weight-on-bit forces the downward pointing teeth of therotating cones into the formation being drilled, applying a compressivestress which exceeds the yield stress of the formation, and thusinducing fractures. The resulting fragments are flushed away from thecutting face by a high flow of drilling fluid.

The drill string typically rotates at 150 rpm or so, and sometimes ashigh as 1000 rpm if a downhole motor is used, while the roller conesthemselves typically rotate at a slightly higher rate. At this speed theroller cone bearings must each carry a very bumpy load which averages afew tens of thousands of pounds, with the instantaneous peak forces onthe bearings several times larger than the average forces. This is ademanding task.

Background: Selection of Insert Shapes

A wide variety of shapes have been used for the inserts ofroller-cone-type bits. These include, for example, hemisphericalinserts, where the exposed surface is generally spherical; pointedinserts, which are also axisymmetric but rise higher, for a given insertdiameter, than hemispherical inserts would; chisel-shaped inserts,having a “crest” orientation; and more complex shapes. Insert design andselection is itself a complex and highly developed area of engineering.

Proper insert selection depends on the formation being drilled. Veryhard formations will typically be drilled with hemispherical inserts;sandstone formations will typically use pointed inserts; and shalyformations will commonly use chisel-shaped inserts.

Drilling with Mixed Tooth Types

The present application discloses bits, rigs, and methods for rockpenetration, using different types of teeth for a single bottomholetrack.

For example, in one class of embodiments, a single row of one or morecones contains both pointed inserts and chisel-shaped inserts.

In another class of embodiments, the same bottomhole track is attackedby inserts of different diameters. (For example, a single non-gage rowof a single cone can include inserts of different diameters.) In anotherclass of embodiments, the same bottomhole track is attacked by insertsof different heights. (For example, a single non-gage row of a singlecone can include inserts which protrude upward to different heights.)This can advantageously be implemented, for example, usinglarger-diameter inserts for the ones which have greater protrusion fromthe cone.

In another class of embodiments, the same bottomhole track is attackedby inserts of different materials. (For example, a single row of asingle cone can include inserts with different carbide compositions.)One particularly advantageous implementation of this is to combinedifferent carbide compositions with different profiles, so that theinserts with the more “aggressive” profile have a moreabrasion-resistant composition, and the inserts with a more“conservative” profile have a more fracture-resistant composition.(Another advantageous implementation is just the opposite, where theinserts with the more “aggressive” profile have a morefracture-resistant composition, and the inserts with a more“conservative” profile have a more abrasion-resistant composition.)

The disclosed innovations, in various embodiments, provide one or moreof at least the following advantages, many related to efficiencies:

-   Physical efficiencies as related to failing multiple types of rock    with one cutting structure containing multiple    features/shapes/extensions/diameters;-   Aggressive as related to addressing multiple types of rock    (soft/hard/sandy/shaley/etc) with one cutting structure (containing    multiple features/shapes/extensions/diameters);-   Durability as related to addressing multiple types of rock    (soft/hard/sandy/shaley/etc) with one cutting structure (containing    multiple features/shapes/extensions/diameters);-   Mechanical efficiencies (WOB/RPM) through related to failing    multiple types of rock with one cutting structure (containing    multiple features/shapes/extensions/diameters).

A further expected advantage, of some embodiments at least, is improvedresistance to secondary tooth fractures induced by a first toothfracture: when more durable teeth are mixed with less durable teeth, themore durable teeth are expected to be more resistant to secondaryfracture.

It should also be noted that the advantageous obtained by the disclosedinnovations can be used in various ways: for example, increaseddurability can be traded off for higher ROP in a given formation, orvice versa.

BRIEF DESCRIPTION OF THE DRAWING

The disclosed inventions will be described with reference to theaccompanying drawings, which show important sample embodiments of theinvention and which are incorporated in the specification hereof byreference, wherein:

FIGS. 1 and 2 show the cone structure of a first sample embodiment, fromtwo different perspectives.

FIG. 3 shows the cone structure of a second sample embodiment. Thisembodiment also combines conical and chisel-shaped inserts in a singlerow, but note that the orientations of the chisel-shaped inserts are notthe same as in the embodiment of FIGS. 1 and 2.

FIG. 4 illustrates some other combinations of different types of teethin a single row.

FIG. 5 shows an exemplary drill rig.

FIG. 6 shows a conventional rotary cone (or “roller-cone”) drill bit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The numerous innovative teachings of the present application will bedescribed with particular reference to the presently preferredembodiment (by way of example, and not of limitation).

The present application teaches combination or mixing of different typesof inserts—whether shapes, diameters, extensions, and ormaterials—within the same cone row, within other rows on the same cone,or in conjunction rows on with other cones which impact a shared bottomhole track—in an effort to enhance drilling performance, either byimproved rates of penetration or durability or a combination of both.These features can be beneficial in transitional formations, mixedlithology or uniform lithology. By varying the above insert variables ina given row or a combination of rows the crater shape, bottom holepattern, and or effective insert penetration can be enhanced throughextra action on bottom, prefracturing, and/or kerfing the formation,thus combining insert attribute efficiencies to provide performanceimprovement.

Naturally the intermesh clearances need to be adequate for the insertwith the most protrusion in each row, and for the insert with thegreatest width.

It is also preferable to check the bit's balance, as described in U.S.Pat. Nos. 6,213,225 and 6,095,262, both of which are hereby incorporatedby reference. (Use of multiple tooth types in a single row means thatmore effort will be required to input the full data needed for theevaluations and optimizations described in these patents, but thoseprocedures are expected to be particularly beneficial in this context.)Manufacturing confusion is another area where the use of the disclosedinventions may require additional care, so that each insert is placedwith exactly the desired DFA, DFB and angle.

A design method which can be useful in connection with these differentinsert shapes is to maximize insert row clearances to maximize insertdiameter, and then altering insert parameters in a give row.

Embodiments Combining Differently-Shaped Inserts

The present application discloses bits, rigs, and methods for rockpenetration, using different types of teeth for a single bottomholetrack.

First Sample Embodiment

For example, in one class of embodiments, a single row of one or morecones contains both pointed inserts and chisel-shaped inserts. In otherclasses of embodiments, inserts of different diameters can be combined,or inserts of different heights, or even inserts of different materials.

FIGS. 1A and 1B show the cone structure of a first sample embodiment,from two different perspectives. In this embodiment the alternatingconical and chisel shaped inserts in a drive row have the same diameter,are made of the same material, and have the same extension. (As will beobvious to those skilled in the art, the “conical” inserts typically donot have a sharp tip, but have a rounded or spherical tip on a conicalbase.)

Note that the gage row itself, in this embodiment, does not have amixture of tooth types. Instead, the driver row (the next row inboard ofthe gage row), and optionally some of the inner rows, have a mixture ofdifferent tooth types.

Also visible are gage buttons, on the backface of the cone, which helpprotect this area.

It is believed that the combination of axisymmetric and chisel-shapedinserts may be particularly synergistic, in that the axisymmetric insertcan efficiently initiate failure of rock which is then efficientlyremoved by the chisel-shaped insert.

Second Sample Embodiment

FIG. 3 shows the cone structure of a second sample embodiment. Thisembodiment also combines conical and chisel-shaped inserts in a singlerow, but note that the orientations of the chisel-shaped inserts are notthe same as in the embodiment of FIGS. 1 and 2.

Other Mixed-Shape Embodiments

The specific combination of conical- and chisel-shaped inserts isparticularly attractive, but many other combinations of different shapesare possible. For example, inserts which have multiple flats, e.g. in anasymmetric shape like that of a cape chisel, can be combined withconical inserts, or which chisel-shaped inserts.

Embodiments Combining Inserts of Unequal Protrusion/Extension

In another class of embodiments, the same bottomhole track is attackedby inserts of different heights. (For example, a single non-gage row ofa single cone can include inserts which protrude upward to differentheights.) The inserts with higher protrusions (greater heights) canaccelerate cutting for as long as they last, which the inserts withlower protrusions provide a more durable and conservative complement.

FIG. 4 illustrates some other combinations of different types of teethin a single row. For simplicity in illustrating the alternation orvariation of teeth, the geometry has been transformed so that theinserts are shown in a straight line. Top and section views are given.

In the example shown, a sequence of four teeth is illustrated:chisel-shaped insert 410, low-protrusion large-diameter “fallback”insert 420, another chisel-shaped insert 410, and a conical insert 430which has the same height as the chisel-shaped inserts 410. In thisexample the full-height conical insert 430 cooperates with thechisel-shaped inserts 410 to achieve rapid cutting in tractableformations, and the more durable insert 420 provides increasedsurvivability. Preferably the more durable insert 420 is given adifferent composition, e.g. of a more abrasion-resistant carbide. Thusthis figure illustrates unequal protrusion (or extension, i.e. heightabove the surface of the cone), as well as unequal diameters, differentmaterials, and combination of more than two different types in the samerow.

Embodiments Combining Inserts of Unequal Diameter

In another class of embodiments, the same bottomhole track is attackedby inserts of different diameters. (For example, a single non-gage rowof a single cone can include inserts of different diameters.) This hasthe advantage that durability can be maximized by large-diameterinserts, without the design inconvenience and crowding which wouldresult from use of large-diameter inserts only.

FIG. 4, as noted above, illustrates an example of mixed insertdiameters.

Embodiments Combining Inserts of Unequal Diameter and Unequal Protrusion

FIG. 4, as noted above, illustrates an example of mixed insert typeswhere both diameter AND protrusion are different between two of thetypes. As noted above, this can be a synergistic combination.

The example shown combines a large-diameter low-height insert with asmaller-diameter and greater-protrusion insert. However, the oppositecombination can also have advantages: a larger diameter can be given tothe insert which has greater protrusion, to reduce the risk of breakagefrom scraping-related forces.

Embodiments Combining Inserts of Different Materials

In another class of embodiments, the same bottomhole track is attackedby inserts of different materials. (For example, a single row of asingle cone can include inserts with different carbide compositions.)One particularly advantageous implementation of this is to combinedifferent carbide compositions with different profiles, so that theinserts with the more “aggressive” profile have a moreabrasion-resistant composition, and the inserts with a more“conservative” profile have a more fracture-resistant composition.(Another advantageous implementation is just the opposite, where theinserts with the more “aggressive” profile have a morefracture-resistant composition, and the inserts with a more“conservative” profile have a more abrasion-resistant composition.)

FIG. 4, as noted above, illustrates this class of embodiments also: notethat insert 420 is hatched differently than the others, to show that ithas a different composition.

Combining Inserts of Different Shapes and Different Materials

It is believed that the combination of axisymmetric and chisel-shapedinserts may be particularly synergistic, in that the axisymmetric insertcan efficiently initiate failure of rock which is then efficientlyremoved by the chisel-shaped insert. In this example of differentiatedtooth functionality, both compositions and shapes of the two types ofteeth can be separately optimized.

In another class of embodiments, the same bottomhole track is attackedby inserts of different materials. (For example, a single row of asingle cone can include inserts with different carbide compositions.)One particularly advantageous implementation of this is to combinedifferent carbide compositions with different profiles, so that theinserts with the more “aggressive” profile have a moreabrasion-resistant composition, and the inserts with a more“conservative” profile have a more fracture-resistant composition.(Another advantageous implementation is just the opposite, where theinserts with the more “aggressive” profile have a morefracture-resistant composition, and the inserts with a more“conservative” profile have a more abrasion-resistant composition.)

Combining Inserts which Differ in More than Two Ways

In further alternative embodiments, at least two types of teeth can bemade different in three or more respects. For example, chisel-shapedteeth which perform scraping in soft formations can optionally becombined with blunt conical teeth with larger diameters and lessprotrusion, for maximum survivability when hard horizons areencountered.

Combining More than Two Types of Inserts

In further alternative embodiments, it is contemplated that three ofmore types of inserts can be combined in the same row (or hitting thesame bottom-hole track). For example, chisel-shaped teeth which performscraping in soft formations can optionally be combined with bluntconical teeth with larger diameters and less protrusion, for maximumsurvivability when hard horizons are encountered.

According to a disclosed class of innovative embodiments, there isprovided: A rotary-cutter rock-penetrating drill bit, comprising: aplurality of rotatable elements, each bearing thereon first and secondtypes of cutting elements which incrementally remove rock as the drillbit is rotated and advanced; wherein at least some of said first andsecond types of cutting elements remove rock from a shared bottom-holelocation, and wherein said first and second types of cutting elementsare differently optimized for different respective formation types.

According to another disclosed class of innovative embodiments, there isprovided: A rotary-cutter rock-penetrating drill bit, comprising: aplurality of cutting elements which incrementally remove rock from acutting face as the drill bit is rotated and advanced; wherein at leastone track of said cutting face is impinged on by first and second typesof said cutting elements having different shapes.

According to another disclosed class of innovative embodiments, there isprovided: A rotary-cutter rock-penetrating drill bit, comprising: aplurality of cutting elements which incrementally remove rock from acutting face as the drill bit is rotated and advanced; wherein at leastone track of said cutting face is impinged on by first and seconddifferent types of said cutting elements, wherein said first type ismore axisymmetric than said second type.

According to another disclosed class of innovative embodiments, there isprovided: A rotary-cutter rock-penetrating drill bit, comprising: aplurality of cutting elements which incrementally remove rock from acutting face as the drill bit is rotated and advanced; wherein at leastone track of said cutting face is impinged on by first and second typesof said cutting elements, and wherein cutting elements of said firsttype protrude deeper into said cutting face than said elements of saidsecond type.

According to another disclosed class of innovative embodiments, there isprovided: A rotary-cutter rock-penetrating drill bit, comprising: aplurality of rotatable elements, each bearing thereon first and secondpluralities of inserted cutting elements which incrementally remove rockas the drill bit is rotated and advanced; wherein at least some of saidfirst and second cutting elements remove rock from a shared location,and wherein said first and second inserted cutting elements havedifferent diameters.

According to another disclosed class of innovative embodiments, there isprovided: A rotary-cutter rock-penetrating drill bit, comprising: aplurality of rotatable elements, each bearing thereon first and secondpluralities of inserted cutting elements which incrementally remove rockas the drill bit is rotated and advanced; wherein at least some of saidfirst and second cutting elements remove rock from a shared location,and wherein said first and second inserted cutting elements havedifferent material compositions.

According to another disclosed class of innovative embodiments, there isprovided: A method for rotary drilling, comprising the actions of:applying torque and downhole force to a weight-on-bit to a bit asdescribed in one of the six preceding paragraphs, while pumping drillingfluid through a drill string to which said bit is connected.

According to another disclosed class of innovative embodiments, there isprovided: A rotary drilling system, comprising: a bit a bit as describedin one of the seven preceding paragraphs, a drill string which isconnected to conduct drilling fluid to said bit from a surface location;and a rotary drive which rotates at least part of said drill stringtogether with said bit.

According to another disclosed class of innovative embodiments, there isprovided: A cutter for a roller-cone-type rock-penetrating drill bit,comprising: a tapered cutter body bearing a gage row, and at least oneother row of cutting elements; wherein said other row includes first andsecond different types of said cutting elements, wherein said first typeis more axisymmetric than said second type.

According to another disclosed class of innovative embodiments, there isprovided: A cutter for a roller-cone-type rock-penetrating drill bit,comprising: a tapered cutter body bearing a gage row, and at least oneother row of cutting elements; wherein said other row includes first andsecond different types of said cutting elements, wherein said first typehas a larger diameter than said second type.

According to another disclosed class of innovative embodiments, there isprovided: A cutter for a roller-cone-type rock-penetrating drill bit,comprising: a tapered cutter body bearing a gage row, and at least oneother row of cutting elements; wherein said other row includes first andsecond different types of said cutting elements, wherein said first typeprotrudes higher from said body than does said second type.

According to another disclosed class of innovative embodiments, there isprovided: A cutter for a roller-cone-type rock-penetrating drill bit,comprising: a cutter body bearing a gage row, and at least one other rowof cutting elements; wherein said other row includes first and seconddifferent types of said cutting elements, wherein said first type andsaid second type have different cermet compositions.

Modifications and Variations

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a tremendous range of applications, and accordingly the scope ofpatented subject matter is not limited by any of the specific exemplaryteachings given. Some contemplated modifications and variations arelisted below, but this brief list does not imply that any otherembodiments or modifications are or are not foreseen or foreseeable.

In various embodiments, various ones of the disclosed inventions can beapplied not only to bits for drilling oil and gas wells, but can also beadapted to other rotary drilling applications (especially deep drillingapplications, such as geothermal, geomethane, or geophysical research).

In various embodiments, various ones of the disclosed inventions can beapplied not only to pure drill bits (as illustrated), but also to otherroller-cone-type rock-removal machines, such as hole reamers, coringbits, or even to large tunnel-boring machines.

In various embodiments, various ones of the disclosed inventions canalso be applied to air-cooled mining-type drill bits.

In various embodiments, various ones of the disclosed inventions can beapplied not only to top-driven and table-driven configurations, but canalso be applied to other rotary drilling configurations, such as motordrive.

In a less preferred class of alternative embodiments, the many proposedvariations in tooth type can also be applied to milled cutters.

In another class of alternative embodiments, the many proposedvariations in tooth type can also be implemented with a matrix conestructure in which the varying tooth types are formed integrally withthe cermet cone.

In many of the embodiments described above, the gage row itself does notinclude multiple types of insert. However, in other embodiments,different types of teeth can be combined in the gage row too (andpreferably also in the driver row and/or other non-gage rows). Insertselection in the gage row is somewhat more constrained than elsewhere,because of the need to trim the gage surface as well as the hole bottom;but subject to this constraint, the disclosed innovations can also beadapted to gage row design.

In another class of embodiments, the same bottomhole track is attackedby inserts of different diameters. (For example, a single non-gage rowof a single cone can include inserts of different diameters.) In anotherclass of embodiments, the same bottomhole track is attacked by insertsof different heights. (For example, a single non-gage row of a singlecone can include inserts which protrude upward to different heights.)This can advantageously be implemented, for example, usinglarger-diameter inserts for the ones which have greater protrusion fromthe cone.

In another class of embodiments, the same bottomhole track is attackedby inserts of different materials. (For example, a single row of asingle cone can include inserts with different carbide compositions.)One particularly advantageous implementation of this is to combinedifferent carbide compositions with different profiles, so that theinserts with the more “aggressive” profile have a moreabrasion-resistant composition, and the inserts with a more“conservative” profile have a more fracture-resistant composition.(Another advantageous implementation is just the opposite, where theinserts with the more “aggressive” profile have a morefracture-resistant composition, and the inserts with a more“conservative” profile have a more abrasion-resistant composition.)Additional general background on drilling, which helps to show theknowledge of those skilled in the art regarding implementation optionsand the predictability of variations, may be found in the followingpublications, all of which are hereby incorporated by reference: Baker,A PRIMER OF OILWELL DRILLING (5.ed. 1996); Bourgoyne et al., APPLIEDDRILLING ENGINEERING (1991); Davenport, HANDBOOK OF DRILLING PRACTICES(1984); DRILLING (Australian Drilling Industry Training Committee 1997);FUNDAMENTALS OF ROTARY DRILLING (ed. W. W. Moore 1981); Harris,DEEPWATER FLOATING DRILLING OPERATIONS (1972); Maurer, ADVANCED DRILLINGTECHNIQUES (1980); Nguyen, OIL AND GAS FIELD DEVELOPMENT TECHNIQUES:DRILLING (1996 translation of 1993 French original); Rabia, OILWELLDRILLING ENGINEERING/PRINCIPLES AND PRACTICE (1985); Short, INTRODUCTIONTO DIRECTIONAL AND HORIZONTAL DRILLING (1993); Short, PREVENTION,FISHING & REPAIR (1995); UNDERBALANCED DRILLING MANUAL (Gas ResearchInstitute 1997); the entire PetEx Rotary Drilling Series edited byCharles Kirkley, especially the volumes entitled MAKING HOLE (1983),DRILLING MUD (1984), and THE BIT (by Kate Van Dyke, 4.ed. 1995); the SPEreprint volumes entitled “Drilling,” “Horizontal Drilling,” and“Coiled-Tubing Technology”; and the Proceedings of the annual IADC/SPEDrilling Conferences from 1990 to date; all of which are herebyincorporated by reference.

None of the description in the present application should be read asimplying that any particular element, step, or function is an essentialelement which must be included in the claim scope: THE SCOPE OF PATENTEDSUBJECT MATTER IS DEFINED ONLY BY THE ALLOWED CLAIMS. Moreover, none ofthese claims are intended to invoke paragraph six of 35 USC section 112unless the exact words “means for” are followed by a participle.

The claims as filed are intended to be as comprehensive as possible, andNO subject matter is intentionally relinquished, dedicated, orabandoned.

1-21. (canceled)
 22. A rotary-cutter rock-penetrating drill bit,comprising: a plurality of rotatable elements, each bearing thereonfirst and second pluralities of inserted cutting elements whichincrementally remove rock rock as the drill bit is rotated and advanced;wherein at least some of said first and second cutting elements removerock from a shared location, and wherein said first and second insertedcutting elements have different material compositions.
 23. The bit ofclaim 22, wherein at least one single row of inserts on at least onerotatable element includes ones of said first and ones of said secondcutting elements.
 24. The bit of claim 22, wherein said cutting elementscomprise diamond-loaded cermets.
 25. The bit of claim 22, wherein saidcutting elements are inserted into said rotatable element.
 26. The bitof claim 22, wherein said rotatable elements are attached through arotary joint to arms which are affixed to a body having an API thread.27. The bit of claim 22, comprising only three of said rotatableelements. 28-32. (canceled)
 33. A method for rotary drilling, comprisingthe actions of: (a.) applying torque and axial force to a bit accordingto claim 22, while (b.) pumping drilling fluid through a drill string towhich said bit is connected.
 34. A rotary drilling system, comprising: abit according to claim 22; a drill string which is connected to conductdrilling fluid to said bit from a surface location; and a rotary drivewhich rotates at least part of said drill string together with said bit.35. A rotary drilling system, comprising: a bit according to claim 22; adrill string which is connected to conduct drilling fluid to said bitfrom a surface location; and a rotary drive which rotates at least partof said drill string together with said bit.
 36. A rotary drillingsystem, comprising: a bit according to claim 22; a drill string which isconnected to conduct drilling fluid to said bit from a surface location;and a rotary drive which rotates at least part of said drill stringtogether with said bit.
 37. A rotary drilling system, comprising: a bitaccording to claim 22; a drill string which is connected to conductdrilling fluid to said bit from a surface location; and a rotary drivewhich rotates at least part of said drill string together with said bit.38-45. (canceled)
 46. A cutter for a roller-cone-type rock-penetratingdrill bit, comprising: a tapered cutter body bearing a gage row, and atleast one other row of cutting elements; wherein said other row includesfirst and second different types of said cutting elements, wherein saidfirst type and said second type have different cermet compositions. 47.The cutter of claim 46, wherein said body is steel, and said cuttingelements are cermet inserts.